In Rotary Damper Torque Calculation, we explained how to estimate the maximum or reference torque required by the application.
But does this mean that once the required torque has been calculated, the correct rotary damper can be selected immediately?
Not necessarily.
For example, suppose the calculated torque is about 2 N·m. There may be many rotary dampers with torque ratings around this value. As we mentioned earlier, rotary dampers can be divided into different types, such as disk dampers, vane dampers, gear dampers, and barrel dampers. Within each type, there are also many different models.
So a calculated torque of 2 N·m does not tell us exactly which damper to use. It only tells us that we should start looking at dampers around the 2 N·m torque range, rather than products rated at 0.2 N·m or 10 N·m.
The next step is to filter these candidates by some basic requirements, such as the damping direction, whether one-way or two-way damping is needed, and the working angle.
Even after these requirements are confirmed, there may still be many dampers that appear suitable. In some cases, a damper may meet the required torque, damping direction, and working angle, but the actual movement of the product is still not satisfactory after installation.
So what should we look at next?
The next step is to understand whether the product’s Gravity Effect Curve and the damper’s Damping Torque Curve work well together throughout the movement.
As we mentioned earlier, the role of a rotary damper can be understood quite simply.
In many opening and closing mechanisms, gravity drives the movement. A rotary damper provides resistance against this movement, helping the product move more slowly, smoothly, and quietly.
When selecting a rotary damper, the first question should not simply be whether your product is a home appliance, a piece of furniture, or an automotive component.
Furniture, home appliances, and automotive products are useful market categories. Products within the same category often have similar structures, so this type of classification is certainly useful.
However, the basic principle when applying a rotary damper is to understand how the force of gravity changes through the movement and how much damping is needed at different positions.
The product’s Gravity Effect Curve describes how the effect of gravity on the product’s motion changes as the product moves through different positions.
The damper’s Damping Torque Curve describes how the damping torque changes during operation. The actual damping torque is mainly affected by factors such as rotational speed and temperature.
The damper’s Damping Torque Curve can also be understood, in a practical sense, as how the damping torque changes as the product moves through different positions. This is because the product does not always move at the same rotational speed throughout the travel. As the product position changes, the damper speed may also change.
When the damper rotates faster or operates continuously, the movement of the internal viscous oil can generate more heat, which may raise the damper temperature. Since both rotational speed and temperature affect damping torque, the torque produced by the damper can change during the movement.
Temperature, however, is not affected by operating speed alone. The surrounding working temperature and other operating conditions can also change the damper temperature. So, in essence, the damper’s torque variation is mainly influenced by rotational speed and temperature, even though we often observe these changes as the product moves through different positions.
In general, when the effect of gravity on the product is stronger, more damping is usually needed. When the effect of gravity is weaker, less damping is usually needed.
The goal is not to make the two curves exactly the same. The goal is to make sure that the product does not move too quickly in one part of the travel or become too slow or hesitant in another.
This is the basic idea of Torque Curve Matching.
1. Horizontal Use
Horizontal Use refers to applications where the product is horizontal when fully closed.
At the beginning of the closing movement, the product is usually closer to a vertical position, where gravity has relatively little effect on its rotation.
As the product moves toward the horizontal position, the effect of gravity on its rotation becomes stronger and reaches its highest level near the horizontal position.
So the relationship is:
Position: Vertical → Horizontal
Gravity Effect Curve: Low → High
Desired Damping Torque Curve: Low → High
The damper should provide relatively low damping torque at the beginning and gradually increase it as the product approaches the horizontal position. This allows the product to close smoothly and quietly.
A toilet seat is a typical example.
When the toilet seat first starts to fall, it is still relatively close to the vertical position, so the effect of gravity on its rotation is small. If the damping torque is too high at this stage, the seat may move very slowly or even hesitate before closing.
As the seat approaches the horizontal position, the effect of gravity on its rotation becomes much stronger. If the damping torque is too low at this stage, the seat may suddenly accelerate and hit the toilet bowl hard.
This not only affects the user experience, but can also cause unnecessary impact on the ceramic toilet body.
2. Vertical Use
Vertical Use refers to applications where the product is vertical when fully closed.
This movement is the opposite of Horizontal Use.
At the beginning of the movement, the product is usually closer to the horizontal position, where gravity has a relatively strong effect on its rotation.
As the product moves toward the vertical position, the effect of gravity on its rotation becomes weaker.
So the relationship is:
Position: Horizontal → Vertical
Gravity Effect Curve: High → Low
Desired Damping Torque Curve: High → Low
The key to this type of movement is to hold the product back at the beginning, and then let it move down slowly and steadily.
Because the effect of gravity is strongest at the start, the damper needs to provide relatively high damping torque immediately. As the product moves closer to the vertical position, less damping is needed, so the damping torque can gradually decrease.
Typical examples include wall-mounted folding tables and folding seats.
If the damping torque is too low at the beginning, the product may suddenly rush downward as soon as it starts moving. This can startle the user and create a serious safety risk. A vertically moving panel can be especially dangerous because its edge may strike the user directly.
For this reason, Vertical Use applications need to be controlled from the very beginning, allowing the product to move downward slowly and steadily until it reaches the vertical position.
3. Horizontal & Vertical Use
Horizontal & Vertical Use refers to applications where the product starts in an inclined position, passes through the horizontal position, and then continues to another inclined position.
Unlike the first two motion types, the effect of gravity does not simply increase or decrease throughout the movement. It increases as the product approaches the horizontal position, reaches its highest level near horizontal, and then decreases again after passing it.
So the relationship is:
Position: Inclined → Horizontal → Inclined
Gravity Effect Curve: Low → High → Low
Desired Damping Torque Curve: Low → High → Low
A slanted trash-bin lid is a typical example.
When the lid is fully open, it is inclined, so gravity has a relatively small effect on its rotation. As the lid moves toward the horizontal position, this effect becomes stronger and reaches its highest level near horizontal.
After the lid passes the horizontal position and continues toward the opposite inclined position, the effect of gravity on its rotation becomes weaker again.
The damper should therefore provide relatively low damping torque at the beginning, increase to its highest damping torque near the horizontal position, and then gradually decrease again.
This allows the damper to match the movement throughout the full travel, rather than providing the right amount of damping at only one part of the motion.
4. Continuous Rotation
Continuous Rotation can be understood using the same basic logic as the previous motion types: look at how the force acting on the product changes with position, and then match it with the appropriate damping torque curve.
A simple example is a product moving vertically downward under its own weight.
In the first three motion types, the product rotates around a pivot. Although the product’s weight does not change, the effect of gravity on its rotation changes as the product moves through different positions.
A straight vertical movement is different.
When a product moves directly from top to bottom, its weight remains the same and the direction of movement stays aligned with gravity. Therefore, the effect of gravity on the movement remains nearly constant throughout the travel.
So the relationship is:
Position: Top → Bottom
Gravity Effect Curve: Nearly Constant
Desired Damping Torque Curve: Nearly Constant
This means that, unlike the previous motion types, the damping torque does not need to increase or decrease significantly with position.
On a Torque vs. Position graph, the Damping Torque Curve is therefore relatively flat.
How Can Linear Motion Drive a Rotary Damper?
The first three motion types are rotational movements, but Continuous Rotation can also be used with products that move in a straight line.
Because linear motion cannot directly rotate a rotary damper, a transmission mechanism is needed.
A common solution is a rack and pinion:
Linear movement → Rack movement → Pinion rotation → Rotary damper rotation
The rack and pinion does not create damping. It simply converts the product’s linear movement into rotary movement so that the rotary damper can control the speed.
What If the Product Is Pushed by a Spring?
Continuous Rotation can also be used when a product is pushed upward or outward by a spring.
In this case, the spring force plays a similar role to gravity in the previous examples: it is the force that drives the product’s movement.
However, there is an important difference.
In a straight vertical free fall, the effect of gravity remains nearly constant. Spring force, however, usually changes as the spring is released.
So a spring-driven movement cannot simply be treated as:
Force: Constant → Damping Torque: Constant
The required damping torque depends on the actual spring force and the mechanism.
For applications without special motion-control requirements, a relatively flat Damping Torque Curve can still be used to control the pop-up speed.
If more precise speed control is required throughout the full travel, the change in spring force needs to be calculated and matched more carefully.
So Continuous Rotation can be summarized as:
Free Fall
Position: Top → Bottom
Gravity Effect Curve: Nearly Constant
Desired Damping Torque Curve: Nearly Constant
Spring-Driven Pop-Up
Position: Bottom → Top / Inside → Outside
Spring Force Curve: Changes During Travel
Desired Damping Torque Curve: Usually Relatively Flat for General Applications
One practical difference between Continuous Rotation and the first three motion types is that the first three usually work within a limited rotation angle.
With Continuous Rotation, the transmission mechanism can continue rotating the rotary damper as the product moves, making it suitable for longer linear travel.
